Production Of Volatile Fatty Acids From Sewage Sludge Mixed With Kitchen Wastes By Anaerobic Fermentation

The pollution from sewage sludge has become a very serious and important environmental problem in China. Production of volatile fatty acids (VFAs) by anaerobic fermentation and further production of value added products by VFAs is one of potential strategies to realize the reuse and resource of the sewage sludge. Addition of kitchen wastes into the sewage sluge can enhance the VFAs production and substrate degradation efficiency during the anaerobic fermentation. In order to characterize the VFAs production from the mixed substrates, this study conducted the fermentation in a 25L fermentor on the basis of laboratory investigation by adding kitchen wastes into the sludge to produce VFAs The research processes and conclusions are as follows:(1) Kitchen wastes were added into the sludge in the anaerobic fermentation with the ratio of kichen wastes and sewage sludge at 1:3. At this ratio the volatile fatty acids production under different concentrations was determined. The results showed that with the addition of the kitchen wastes, the accumulation of VFAs was up to 17.62 g/L. Based on an overall consideration of various factors, such as the VFAs production, substrate degradation and the production rates,130 g/L of TS concentration was considered the optimal fermentation substrate concentration to produce VFAs in a larger scale process. Additionally, the Monod equation was used to fit the dynamic process during anaerobic fermentation under different substrate concentrations and equation:μ=(0.114·S)/(281.764+S)(Ks:281.764 g/L,μmax:0.114 g/h) was obtained.(2) Sewage sludge and the mixture (sludge and kitchen wastes) were pretreated by high-voltage pulsed discharge plasma systems (plasma) and thermo-alkaline respectively. The changes of substrate characteristics and composition were observed under different pretreatments. Pasma pretreatment changed the characteristics of sludge. However,it may be most attributed to the rising temperature in the system. The solubilization rate of TS, VS, SCOD are 42%,62% and 72.5%, respectively, when the mixture (sludge and kitchen wastes) substrates are pretreated by thermo-alkaline. The solubilization of dissociative protein is 64% and 70.8% respectively when the sludge and mixture (sludge and kitchen wastes) after pretreatments. Meanwhile the solubilization of dissociative carbohydrate could exceed 50%. The solubilization of intracellular material revealed that the sludge through the thermo-alkaline pretreatment released more intracellular proteins (0.019g/L), while the mixture (sludge and kitchen wastes) after hot alkaline pretreatment released more intracellular carbohydrate (0.014 g/L).(3) The production of VFAs during fermentation after different pretreatments with pure sludge or mixed sludge was studied, respectively. After the heat-alkaline pretreatment, the VFAs from the sludge and the mixture (sludge and kitchen wastes) have increased by 6.81 g/L and 3.30 g/L, the production rates have increased by 0.19 g/gVS and 0.23 g/gVS. The main products in VFAs was acetic acid during the sludge fermenting, but we got acetic acid and butyric acid in the VFAs more than other single acids when fermented the mixture (sludge and kitchen wastes). In the process of fermentation, protein were more exploited than carbohydrates, both of them with the dissociative form were the major source of product VFAs, but the free protein was the main contribution source. (4) The method of filtration and centrifugation were used to separate the fermentation liquid after thermo-alkaline pretreatment. The effects of separation about different methods process (pretreatment, fermentation and sepration) were evaluated. The results revealed that the effect of centrifugation was better than filtration; the moisture content within solid was above 80% after filtration while the moisture conten was below 80% after centrifugation.The effect of separation about the mixture (sludge and kitchen wastes) was better than the sludge. We got the highest yield of VFAs as 68.93% when the fermentation broth of the mixture (sludge and kitchen wastes) after centrifugation.